Myocardial infarction, commonly known as a heart attack, is one of the leading causes of mortality in the western world. Myocardial infarction occurs when the blood supply to part of the heart muscle is interrupted causing some of the heart cells to die. The coronary arteries supply the heart muscle with the oxygenated blood that the heart needs to function. When the coronary artery flow is restricted heart cells will not get the oxygen they need to function, and could die. As the myocardial cells die, the ability of the heart to pump blood throughout the body is impaired.
Early detection and prompt intervention of an acute myocardial infarction significantly improves the clinical outcome. The mean time from myocardial infarction symptom onset to arrival at a hospital for treatment is about 2.5 to 3 hours. Surveys and focus groups of heart patients, family members, and the public report that they thought the presenting symptoms were less dramatic then expected. Many patients take a “wait and see” approach until they are more certain of the symptoms significance. A large proportion of irreversible myocardial injury and fatal ventricular arrhythmias occur in the first several hours after closure of a coronary artery. The longer the time between closure of the artery and the treatment lead to further myocardial necrosis and worse clinical outcomes. Treatment could include a defibrillation shock if the patient has an irregular heart arrhythmia, medication to help dissolve blood clots and open a blocked coronary artery, or a stent placement to open the blocked coronary artery.
Ischemic heart disease, or myocardial ischemia, is a disorder caused by a critical coronary artery obstruction termed atherosclerotic coronary artery disease (CAD). Atherosclerosis occurs when fatty material and plaque buildup on the walls of the artery. The buildup narrows the artery and the blood flow is decreased potentially leading to myocardial infarction. Myocardial infarction is an acute form of ischemic heart disease. Myocardial ischemia may be temporary and reversible, or permanent and irreversible leading to myocardial infarction. Myocardial ischemia can be temporary when there is brief periods of coronary occlusions followed by reperfusion, possibly in situations when the coronary blockage is not significant enough during normal conditions but as demand increases, for example with exercise, the blockage could become significant until the demand returns to normal. This has been termed myocardial stunning. Reversible myocardial ischemia depends mostly on the amount of time the myocardial cells have been restricted of oxygen, the shorter the amount of time the better, from seconds to minutes. The damage becomes permanent and irreversible when the blockage is severe or the flow has been compromised for longer, from minutes to hours.
Diagnosis of myocardial ischemia prior to a heart attack is important for optimal disease management. Approximately 1.5 million Americans a year have a heart attack resulting from myocardial ischemia, of these approximately 500,000 are fatal. In one third of those patients, CAD is not diagnosed until after a heart attack occurs. Treating known CAD is beneficial. Coronary artery revascularization, such as angioplasty, coronary artery bypass graft (CABG), or stent placement, and other medical therapies, such as medication, significantly reduce the morbidity and mortality rates of this disease. Therefore, early detection and diagnosis of CAD is critical.
The diagnosis of CAD is difficult because in many cases the disease is not apparent until after the patient has had a heart attack. Some patients with CAD experience symptoms such as unstable angina while others have no symptoms at all. This disease occurs in a range of patients, both the young and old, women and men, and in patients with and without co-morbidities. There currently is not a uniformly accepted screening method for CAD but the most common forms of testing are treadmill, or stress testing, and for patients with moderate-to-high risk for CAD, a cardiac imaging study.
Myocardial ischemia results in electrophysiological changes that are detectable. Within seconds of the onset of myocardial ischemia, there are ventricular morphology changes, including ST segment changes, called an ST shift, which can be seen on a surface electrocardiography (EKG).
Myocardial ischemia impairs ventricular contraction and relaxation, therefore altering the ST segment. In general, the underlying cause of the ST shift is an altered ion transport across the myocardial cell membrane. Measuring the ST segment deviation on the surface EKG, usually using a caliber-based technique or automated system, is the most common clinical technique for diagnosis of myocardial ischemia.
Treadmill tests are used on patients with symptoms or signs of CAD and patients with significant risk of CAD. Surface EKG monitoring is used during treadmill tests to see if there are any morphology changes in the beat complex, such as ST changes, during exercise when the heart requires more oxygen. Another method for detecting myocardial ischemia is with long-term electrocardiography recording using a Holter EKG monitor.
Holter EKG monitoring can help in detecting both symptomatic and asymptomatic, or silent, myocardial ischemia and is fundamental for characterizing episodes in patients with suspected or documented CAD. Long-term monitoring is beneficial since studies have shown that some patients, particularly with angina, experience short myocardial episodes at night or in the morning. Therefore, these patients' treadmill tests are usually negative. Holter EKG monitoring may reveal ischemia in about 10% of those with a negative treadmill test. Holter EKG monitoring has also shown that episodes of subendocardial (occurring under the endocardial, the inner most layer of tissue, of the wall of the heart) ischemia have a typical circadian distribution with a first peak in the morning hours and a second peak in the afternoon. Many studies have indicated that ischemia, even transient ischemia, on Holter EKG monitoring are among the major predictors of cardiac events in patients.
Practical application of long-term monitoring for ST segment deviations have been limited, due in part to the inconvenience of Holter EKG monitoring and the prevalence of false-positive ST segment deviations because of noise, postural changes, and artifacts. Chronic ischemia monitoring would be very beneficial in documenting the reproducibility of the ischemic pattern since day-to-day episodes can be variable. In addition, early and reliable detection of myocardial ischemia would be clinically valuable if the result is an improvement of the time to treatment for patients.
Another approach to chronic monitoring of the ST segment deviations is to use intracardiac electrograms (IEGMs) that are recorded by permanently implanted electrodes used in pacemakers and implantable cardioverter defibrillator (ICD) devices. IEGMs avoid the insulating effects of the lungs and thorax due to the distance between the electrodes, giving a five to 10 times larger signal amplitude then a surface EKG. Noise and signal artifacts are also greatly reduced due to the lack of electrode-skin interfaces. The location of the implanted electrodes is convenient, consistent, permanent, and capable of continuously monitoring of the ST segment using the IEGM. Indication for an ICD implant generally revolves around patients having or being ask risk for ventricular arrhythmias and about 50% of ICD patients either have documented CAD or are at risk of developing CAD. Several studies have shown that cardiac ischemia can be detected from IEGM of an ICD and that the sensitivity of IEGM for detection of ischemia may be superior to that of surface EKG. Another advantage of using implanted cardiac rhythm management devices is the ability to correlate ST segment changes with other cardiac events, such as ventricular arrhythmias.
The immediate benefit of chronic ST segment monitoring is the early confirmation of an acute coronary event or acute myocardial infarction in patients who have an implanted device. A second benefit is the potential to notify patients upon detection of a significant coronary event. This could greatly decrease the amount of time between ischemia onset and treatment. Additionally, the monitoring of the ST segment provides the possibility of early intervention that could positively affect clinical outcomes in CAD patients who have changes in their ischemia profile over the course of their disease.